The invention relates to a process and a device for producing a gas mixture which contains at least one gaseous component, in particular for producing a calibration gas.
The use of gases with defined concentrations of admixtures is required both in various technical devices and as calibration standards for analysis methods.
In all analytical devices, calibration represents the most important step with a view to obtaining correct results from the analysis. Although the apparatus settings of an analysis device may have a decisive influence on the sensitivity and accuracy of an analysis result (and therefore the detection limit), correctness can only be ensured by comparison with a calibration specimen with a known content.
In the environmental analysis field, the need for global, time-independent comparability of analysis results, means that correctness is a crucial parameter if it is to be possible to understand ecological processes and understand global mass streams. Nowadays, the analysis of gaseous pollutants down to the lower ppt range no longer constitutes any problem for many analysis devices, for example those used in gas chromatography. However, the problem of appropriate calibration of such analyses remains substantially unresolved.
A continuous stream of the calibration gas is required in order to filter out any conditioning phenomena. The requirements of trace analysis of gaseous pollutants make continuously flowing certified test gases from pressurized-gas vessels the optimum calibration medium. The high price, caused by complex stabilization and production, and the long delivery times, which are caused by production factors, and also the high minimum volumes of the individual vessels make these processes appear too expensive for many users. Moreover, not all gas mixtures can be stabilized in corresponding pressure systems, with the metal surfaces which they employ. In particular, polar components can only be produced in stabilized form with very great difficulty by this route.
An ideal calibration specimen generally contains an accurately defined concentration of the analyte, distributed as homogeneously as possible in the same matrix gas which also surrounds the analysis specimen. However, a calibration specimen of this type can rarely be achieved, in particular in the field of environmental analysis, since it is almost impossible to obtain the uncontaminated matrix required for production of the calibration specimen as a blank and dilution medium.
There is another problem with regard to the analysis of gaseous specimens. Although in this case it is relatively easy to obtain the uncontaminated matrix, for example synthetic air, in this case the production of corresponding standard specimens represents a problem.
Gravimetric methods of producing gas mixtures are extremely complex and can only be undertaken by manufacturers of test gases with considerable financial outlay. This results in problems on account of the masses of the vessels which hold the gas mixture, which are high in relation to the matrix gases, but very particularly in relation to the trace components.
In the case of volumetric production, it must be possible for small volumes of a trace component, which is often in liquid form, to be introduced reproducibly into a very large volume of the matrix gas. There are problems with the stability of the gas mixtures on account of the low density of the gaseous analyte. Adsorption and desorption on the surfaces of the equipment which is in contact with the test gas, which surfaces are very large in relation to the mass of the trace components, lead to the risk of the concentration changing, for example on account of wall desorption and wall adsorption effects.
Commercially available calibration systems which attempt to achieve this have various weak points:
a short operating range of at most two decades,
problems with low concentrations,
limits on the number of components which can be mixed and their ratios,
the minimum achievable concentration range is excessively high.
It is also known for the pure components to be introduced continuously into a continuously flowing gas stream and to be reproducibly homogenized therewith. In this case, critical nozzles, permeation devices or controlled incoming flows of test gases of higher concentrations may be suitable for introduction. However, this does not solve the above problems.
Since, by definition, the detection limit of all analytical devices is decisively dependent on the reproducibility of the individual results, any improvement in the calibration also improves the detection limit and therefore considerably increases the capacity of the overall analysis device.
DE 198 58 366 A1 has disclosed a process of the type described in the introduction in which a capillary diffusion metering system is used in order to establish a defined mixing ratio of a carrier gas and the components which are of interest. Then, the mixture is passed over a trap, through which a purge-gas stream is passed, the components which are of interest being transferred into this stream. However, the accuracy which can be achieved is by no means sufficient for some applications.
U.S. Pat. No. 5,400,665 discloses a process for producing an analysis gas stream in which a liquid mist comprising small drops of a sample which is to be analysed is produced, solvent being eliminated, before the mist is introduced into a carrier gas stream, by heating the mist. In this case, the mist droplets are produced by means of a piezoelement. Apart from the fact that the mist does not contain substantially uniform droplets, but rather, at a frequency of 1.3 MHz, 70% of the droplets are smaller than 13 xcexcm, with the larger droplets being separated out under the force of gravity, this process is not suitable for the production of a gas mixture which contains at least one gaseous trace component in a predetermined concentration, especially since there is no quantitative metering of the sample material.
It is an object of the invention to provide a process which makes it possible to prepare a gas mixture with a known concentration of volatile components on-line even. It is a further object of the invention to provide a process which allows to prepare a gas mixture with a very low concentration range of volatile components. It is still a further object of the invention to provide a process for preparing a gas mixture usuable as calibration standard or for industrial processes.
It is also an object of the invention to provide a device which makes it possible to prepare a gas mixture with a known concentration of volatile components on-line even and especially one having with a very low concentration range of volatile components.
The invention concerns a process for producing a gas mixture which contains at least one gaseous trace component in a predetermined concentration, comprising the steps of:
continuously conveying a matrix gas stream with a predetermined quantitative flow;
introducing at least one trace component into the matrix gas stream in a predetermined quantity per unit volume of matrix gas;
wherein the at least one trace component is introduced in the form of successive microdroplets of substantially the same size and is evaporated in the matrix gas to form the gas mixture, the microdroplets being delivered from at least one nozzle, which is in each case made to contract by means of a piezoelement which is triggered by metering pulses which are generated according to an intended delivery quantity per unit time.
The invention further comprises a device for producing a gas mixture which contains at least one gaseous trace component in a predetermined concentration, comprising a matrix gas source, downstream of which there is a mass flow controller, and at least one source for a component, and a metering device for the at least one trace component, wherein the metering device is a microdroplet-metering device with at least one nozzle which releases successive individual microdroplets, opens out into an evaporator tube, through which the matrix gas flows, and can be made to contract, so as to produce a microdroplet, by means of a piezoelement which is triggered by metering pulses generated according to the intended delivery quantity per unit time.
In this arrangement, a matrix gas stream is conveyed continuously, with a predetermined quantitative flow, and at least one calibration component is introduced into the matrix gas stream in a predetermined quantity per unit volume of matrix gas, the at least one calibration component being introduced in the form of microdroplets of substantially identical size and being evaporated in the matrix gas to form the calibration gas or mother mixture, the microdroplets being delivered by nozzles which are in each case made to contract by means of a piezoelement which is triggered by trigger pulses which are generated according to the intended delivery quantity. This results in a continuous calibration-gas stream with a known concentration of volatile, in particular highly volatile components, in particular under standard pressure. It is possible for dilute gas mixtures comprising a very wide range of components in a very wide range of concentration ratios to be produced on line and made available to the sample introduction system for a gas analysis device which is to be calibrated. If appropriate, the calibration component can be cooled in order to be in the liquid state so that it can be introduced as microdroplets.
Either a solution which contains the components, with one metering head, or a plurality of metering heads amounting to the same number as the number of components, can be used to produce multicomponent gas mixtures.
In this way, it is possible to ensure a highly accurately controlled introduction of a sample into a continuous gas stream and, thereby, to offer ideal conditions for a flexible calibration system. The quantities of analyte substance which are required for calibration are introduced into a carrier gas stream (in particular N2 or synthetic air) which is controlled by an accurate mass flow controller from one microdroplet-delivery device or preferably from a plurality of microdroplet-delivery devices which in each case supply different components. Separate actuation of each microdroplet-delivery device makes it possible to achieve a very wide range of mixing ratios for the component in the gas phase. Moreover, on account of the small droplet size which is to be achieved (in the range from, for example, approximately 20 to 100 xcexcm, in particular approximately 30 to 50 xcexcm), rapid and complete evaporation of the components in the carrier gas stream is ensured. This leads to a homogeneous calibration or test gas. The possibility of introducing 1 to 2000 drops/sec of a volume of approximately 15 to 65 pl of each component into a given gas stream alone allows calibration over more than four concentration decades without the device having to be mechanically altered. Therefore, a process of this type has a considerably greater flexibility with regard to concentration range and component ratios than the processes or test gases which have hitherto been commercially available.
In this way, the accuracy of the analysis results on defined standard samples and also of real samples can be increased considerably, at low cost, compared to conventional calibration techniques, and therefore the reproducibility and, as a result, the detection limit can also be lowered greatly. Furthermore, the calibration gas is available for rapid use, for example in the laboratory.
After the droplet size has been determinedxe2x80x94by gravimetry or by determining the flow rate in a capillary sectionxe2x80x94the calibration generation can be set digitally and therefore in a readily automated mannerxe2x80x94by digital setting of the drop frequencyxe2x80x94to the desired test gas concentration. Moreover, during the sequence, this concentration can be digitally altered at any time, in order to generate an automatic calibration series of different test gas concentrations. A further advantage is that it is also possible to generate test gases of polar components, which cannot be obtained in commercially available pressurized-gas vessels.
The uniform size and sequence of the drops ensure that a homogeneous gas mixture is formed, as is optimum for calibration purposes. Digitally controlling the microdroplet-metering device allows from 1 to 2000 drops with a volume of 15 pl (with a nozzle diameter of d=30 xcexcm) of each component to be introduced into a given gas stream. This allows calibration over more than four decades without the system having to be mechanically changed. To obtain an optimally calibrated gas mixture, the size and homogeneity of the individual drops formed are of crucial importance. If the droplets are of optimum size, the final liquid volume by which the total quantity of trace components is distributed in the matrix gas is dependent solely on the homogeneity of the size distribution. Production of the microdroplets is distinguished by an extremely reproducible method of operation, and the scatter of the individual drop sizes, based on the mass, is approximately 1%, i.e. well below the scatter of comparable processes. The fact that the calibration components are introduced into the matrix gas as microdroplets and not, for example, as a film of liquid leads to the evaporation process taking place highly reproducibly given the same size of droplet, so that a homogeneous gas mixture can be formed.
The precise volumetric flow of matrix gas required in the calibration unit can be ensured by mass flow controllers. These control instruments are expediently used on the matrix gas side to allow ideal calibration thereof and to eliminate any possible disruptive influences from the trace components, which are often reactive and therefore corrosive.
The increases in volume which are to be expected from the evaporation of the trace components can be determined by calculation and taken into account when controlling the microdroplet-metering devices.
The process can be used universally for all analysis methods employed for highly volatile components (for example VOCs and VVOCs). These gas analysis methods are becoming increasingly important in the application areas of instrumental analysis, which are becoming increasingly important such as for example environmental analysis (climate protection) or analysis in the field of health and safety at work. Calibration gases which have been produced in accordance with the invention can be used in gas chromatography devices, gas phase adsorption and desorption devices, in infrared spectroscopy, in UV/VIS spectroscopy or in gas-phase monitoring devices.
As well as applications in chemical analysis, further fields include areas in which ideally defined gas phases can be used, such as climatic chambers, sensor test stands or vapour phase epitaxy systems. The process can also be used to provide, for example, certain gases, such as natural gas, or atmospheres, such as air, with a defined fragrance content.
The quantitative determination of the various gaseous pollutants in atmosphere and ambient air has considerable importance both for climate protection and for health and safety at work. Especially in an industrial society, the ecological effects of gaseous pollutants are of considerable importance, on account of their high diffusion rates and the wide-ranging ways in which they can be conveyed in the environmental compartment air for introduction into the ecosphere. Therefore, contamination of the environmental compartment air by a very wide range of gaseous pollutants play an important role particularly with regard to direct damage, but also to increased levels in ecosystems. In this context, correct analysis results in particular of individual trace components are of particular interest, since these components often have highly damaging effects on ecosystems on account of their ecotoxicological action. The harmful action of highly volatile hydrocarbons (VVOCs and VOCs) are only one example. Accuracy of analysis of these substances, which can often accumulate in ecosystems on account of persistence, has to be constantly improved. Only in this way can all introduction paths, including the natural, low-concentration paths be recorded correctly on a global basis even over prolonged periods of time.
Further objects, embodiments and advantages of the invention are to be found in the following description.